Your RPMs for a Vintage 1915 engine??

So what is the consensus on how many RPMs you Vintage owners will
push your mexicrates to? I know that the VW manuals said that the maximum speed and th cruising speed for the '74s for example were 82 or something like that. Since the VS Speedster weighs a lot less than a Bug, maybe higher revs are ok?

On mine I have never seen the rpms over 3,300 as I am concerned that my engine last as long as possible. 3,300 gives me maybe 70 MPH and I haven't gone over that. Ever in 1,080 miles so far.

I read some crazy rpm numbers like 4,500-6,000 and can't imagine pushing my engine to that amount.

Am I nuts to stay below 70 mph or is it ok to push it over that?

Jack

I'm not familiar with the Mexicrate engines....My understanding is they are displacement enlarged stock crankshaft units....

If memory serves, the Mex. engines came with non counter weighted cast cranks for a number of years...These cranks might have a higher failure rate than the German forged part.....

I've pushed the stock German units to 4,500rpm fairly regularly without radicaly reduced life span...

If your crank is cast, I would reccomend 4,000rpm as a limit...

Larry Jowdy could probably give you more accurate details...

Good Luck....

Leon C.

On the highway, I would cruise with mine at around 4000 or so (maybe a bit more like 4100). I never really pushed it higher than that. The longer I owned the car, the higher my revs seemed to be :)

The engine in the Beetle, currently being stuffed into my Ghia, is a little (1641) AJ Sims engine. His heads, barrels, carbs and cam with a lightened flywheel, DMS forged crank, and balanced internals.

It regularly bounces against the 7,000 RPM rev limiter without a whimper. Has for a couple of years now. The only problem at all is that the revs cause the belt to turn inside out every once and again.

I am also curious like Jack. My questions is not so much cruising speed, it's when shifting gears. I have been short shifting durring the break in. Am I going to hurt anything by going near or over 4000 RMPs. Redline says about 5500.

Marty

If your worried just buy a rev limiting rotor button. They make um with differnent limits for differently built engines. I got one t safe guard the automatic's second shift range.

If I'm not mistaken on those engines the redline is about 5K to 5.5K rpms.

Well I calledKirk at Vintage Speedsters--as usual hewas very helpful.

First I was happy to hear my crank is forged and not cast. kirk said to hold it to 4,000 rpms which should give me over 75 mph. I can easily live with that.

--Hope this helps others.---Jack

Don't forget the other end of the spectrum: one should be mindful not to lug the engine, which is to say ask it to accelerate when running at low RPM, say much below 2500. I'd think of 2,000 as the absolute bottom. Of course, this will all depend on the sort of cam in the engine, and there may be many variants possible. But generally, these engines like to run up, not down. Damage to the bearings can result from loading an engine going too slow.

And you may wonder how/why the bearings might be damaged at lower RPM's, so I'll tell you two words: Power Pulses

As the RPM's drop down below 2,500 rpm and the crank is turning slowly, the air/fuel mix is ignited in the cylinder, the piston is forced down with great deal of force and it pushes the conecting rod firmly against the crankshaft thereby applying all of the force against the closest surface of the bearing. Do this often enough at slow speeds and the rods will try to squash the crank/rod journals in that one spot and flatten it relative to the rest of the circumference of the bearing. Not good. If the rpms are higher (like 3 grand and up) the crank is spinning fast enough to aply the force over more of the bearing circumference, thereby lessening the force in one spot.

Now, this used to be a BIG problem for the few years when Porsche was running roller-bearing crankshafts, and their mechanics and drivers would never, ever run below 3,000 rpm (some of them seldom seemed to get below 4 grand, but that's another story). If the engine lugged with a roller bearing in there, the power pulses would pulverize the needles (rollers) and the bearing would very quickly become junk. With today's technology of babbit bearings, the problem is far less severe and the crankshaft bearings will take a lot more abuse (even if they take more effort to turn than with roller bearings).

So where does that leave us? Well, as mentioned in a post above, try to keep your revs up above 2,500 RPM (in the "green zone" on most of our tachs) and you'll be fine, and remember that today's crank bearings can take it, even if you sometimes drop down to 2,000 or so. If you're running a stock valve train with stock (i.e. "soft") valve springs then 5,000 should be your red-line limit (think of 4,000+ as the beginning of the "yellow-caution" range). Running more HD components and really stiff valve springs (with corresponding chromoly push rods) means that you could run higher RPM's, like up into the 6,500+ range, safely.

BTW: With a cruising limit of 4,000 per Kirk, you should, depending on transmission gearing, be running between 75 and 80 mph - plenty for most of us, and it should be able to safely run all day at that speed (if it doesn't overheat) (another story!!)

OK, as long as we're getting all techy here, one should also mention that in reality, it is the oil that makes your car go. [Huh, what did he say??] Just like you could think that it is the tires that make the car go because it is the rubber meeting the road and the shear force applied at that patch that allows it all to happen. Inside the engine, if you are doing it right, the crank and the conn rods will never meet because it is the oil that keeps them apart. Just like the brick mason who was aked what he uses to keep the bricks together, and who replied that actually he used mortar to keep them apart. These journal bearings are designed to operate w/ a thin layer of oil between the two metal parts, and the amount of pressure that this film can withstand before it collapses is a direct function of the rotational speed of the parts. The faster the oil is sheared between the two surfaces, the more radial pressure it can bear. To some degree, it is the oil pressure built up in these bearings that contributes to the system oil pressure, as this is what the oil pump needs to overcome if the oil is to circulate through the bearings. It is this radial pressure transferred from the conn rod through the oil that ends up as torque at the crank and motive force where the aforementioned rubber meets the road. Fun stuff, huh?

Roger that. My work here is done . . .

Holy $#|+, Batman!!

Sounds just like my Physics 201 Prof., Dr. Lanza, at Northeastern in 1970!!!!!! This guy was really cool.....not only could keep a bunch of Freshmen entertained (and educated) in an amphitheater context, but he was also the head of the Ski School at Loon Mountain, NH! Ok, so he drove a Citroen something-or-other, but he was cool, just the same.

Nevertheless, yes....it's the shear strength of the oil film that hydraulically transfers the force from the connecting rod to the crankshaft journal and THAT, since the oil doesn't compress well under pressure, transfers the force to the bearing and THAT causes the bearing to break (of it's a roller bearing) or distort (if it's a babbit bearing) and there are a multitude of multi-directional forces going on to make it all happen in real time. There...

Now....having said all that, just remember, Speedstah Guys: Don't let the revs of a VW-based engine drop below 2,500 rpm.

Engineers.....ya gotta love'em........but then I always say; I don't have to be an Engineer. I married one and put another one through College....

Somebody (Musbjim?) already said it: You learn the darndest stuff on this forum!!

gn

Here are shift points from a 1979 VW Beetle owner's manual

1st gear up to 16 mph
2nd gear 11-34 mph
3rd gear 22-56 mph
4th gear from 31 mph up

My 1914 (when it was IDA'd, not turbo) regularly saw 7400 at the track in a beetle - but it was built to be able to handle that.

Just by "feel" I figure my redline is about 4600 RPM, this is based on my crapola "vintage reproduction" tach though. As far as sustained cruising speed I think my motor/gearing likes to be kept under 80 mph, although I've done bursts of 100+.

My 2,110 is built to take it to 7 grand and over, but I usually shift somewhere between 4,000 and 5,000 (whatever feels good).

I've been known to hold second and third to over 6,000 (while attempting to pass a Carerra C4 on a road course) but that's pretty rare !>)

gn

I almost passed him!